The receptor concept, now more than century old, remains the core concept in understanding the mechanisms of disease causality and drug action. Originally formulated in the early 1900s, receptor theory has evolved in both detail and complexity as the tools of molecular biology and increasingly sophisticated research technologies have facilitated the study of drug/ligand receptor interactions at the molecular level. The result has been a more detailed and nuanced understanding of the parameters of drug action that together with knowledge of the pleotropic nature of cellular targets and associated signaling pathways has reshaped the receptor concept. Added to the basic agonist/antagonist view of drug/ligand function, concepts like allosterism, inverse agonism, biased signaling and compound residence time have provided a broader understanding of compound-target interactions to better inform drug discovery efforts. The iteration of descriptive data from pharmacological experiments to provide generic indices of affinity and efficacy can now be used to predict expected drug/compound effects in other cell/organ systems where the sensitivity of the latter to drug effects (either via expression levels of cell surface receptors or variances in the efficiency with which those receptors are coupled to cytosolic metabolic processes) can be accommodated.